[R] dissolve tiny polygons to others with unionSpatialPolygons{maptools}

[Kumar Mainali]

Thank you, Roger. I cannot fix the upstream processes to eliminate
generation of these problems. So, I need to deal with the data I got.
I tried various online resources including
https://gist.github.com/mstrimas/1b4a4b93a9d4a158bce4 and tried:
setScales()
set_RGEOS_dropSlivers(TRUE)
set_RGEOS_polyThreshold()

before performing:
gUnaryUnion(p.ranges, id=NULL)

This eliminates the slivers but it also dissolves ALL the polygons to get a
single polygon. That is not something I wanted. I would like to dissolve
only the tiny polygons to the neighboring bigger ones.

Any help is very much appreciated!

ᐧ

Postdoctoral Associate
Department of Biology
University of Maryland, College Park

On Mon, Jun 6, 2016 at 9:24 AM, Roger Bivand <Roger.Bivand at nhh.no> wrote:

> On Mon, 6 Jun 2016, Kumar Mainali wrote:
>
> I am trying to use unionSpatialPolygons() of maptools to eliminate sliver
>> in species range. I want to dissolve tiny sliver polygons in a shapefile
>> to
>> bigger polygons as "Eliminate (Data Management)" of ArcMap does. Whereas I
>> can dissolve polygons that have identical features in the argument "IDs",
>> I
>> cannot dissolve tiny polygons based on some threshold in area. In fact,
>> the
>> argument "threshold" has no effect in the output.
>>
>
> Indeed, threshold is not passed through, it was used when the function
> used gpclib rather than rgeos; I'm minded to deprecate
> maptools::unionSpatialPolygons anyway. Note that the data are in
> geographical coordinates, which may very well not be appropriate for the
> topological operations you are trying to do. Use rgeos::gUnaryUnion
> instead, and refer to this thread:
>
> https://stat.ethz.ch/pipermail/r-sig-geo/2015-November/023667.html
>
> for using rgeos::set_RGEOS_polyThreshold() and friends. They do not,
> however, try to guess which of the polygons neighbouring the sliver should
> get the extra area, so you'll have to think that through yourself.
>
> Googling for lists:R-sig-geo dissolve slivers gets a fair number of hits.
>
> There is always also the upstream question of where the slivers came from,
> and whether the resolution is not in the earlier process - generate a map
> without slivers that says exactly what you mean, rather than fudging it
> afterwards.
>
> Roger
>
>
>> ​Input data is available here: ​
>> https://www.dropbox.com/sh/a0x5bbo9u60y7is/AAB6RjXHFQKZv-i-t4JclF3ba?dl=0
>>
>> p.ranges <- shapefile{raster}
>> (IDs <- p.ranges$style_id)
>> library(maptools)
>> unionSpatialPolygons(p.ranges, IDs = IDs, threshold = 1.5)
>>
>> ​-- Kumar Mainali
>> Postdoctoral Associate
>> Department of Biology
>> University of Maryland, College Park
>>
>>
>> ᐧ
>>
>>
> --
> Roger Bivand
> Department of Economics, Norwegian School of Economics,
> Helleveien 30, N-5045 Bergen, Norway.
> voice: +47 55 95 93 55; fax +47 55 95 91 00
> e-mail: Roger.Bivand at nhh.no
> http://orcid.org/0000-0003-2392-6140
> https://scholar.google.no/citations?user=AWeghB0AAAAJ&hl=en
> http://depsy.org/person/434412

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